1. Field of the Invention
The present invention relates to a method of purifying waste gases, and also to a gas burner.
More specifically, the invention relates to a method of purifying waste gases that originate from a gas burner or some other combustion source, such as oil-fired burners or the exhaust gases from internal combustion engines. The invention also relates to a gas burner in which the method is applied.
2. Description of the Related Art
It is usual to heat industrial furnaces with gas burners. A normal fuel in this respect is natural gas, although other gases such as propane, butane and bottled gas can be used.
In the case of one efficient gas burner, the burner is of the kind in which the burner head is placed on one end of an inner tube around which there is placed an outer, protective tube which is closed at the bottom. The gases or fumes from the burner chamber pass inside the inner tube and down towards the bottom of the outer tube and flow from there in an opposite direction between the outer tube and the inner tube and thereafter into an exhaust passageway that leads to the surroundings. Heat is delivered by the protective tube to a furnace space, this heat comprising 30% convection heat and 70% radiation heat.
Another similar type of burner also includes an inner tube on which there is placed an outer protective tube. In this case, however, the bottom of the protective tube is not closed. The protective tube has an arcuate shape, e.g. a U-shape, with one free end of the protective tube connected to an exhaust passageway. The inner tube with the burner head is straight and is thus located within the straight part of the protective tube.
Such gas burners deliver high concentrations of hydrocarbons (HC) and nitrogen compounds (NOx).
It would be desirable to be able to increase the temperature of the outer tube to about 1155-1200xc2x0 C., so as to enhance the burner power concentration. This can be achieved by making the outer tube from a high temperature material, such as silicon carbide (SiC) or from a material in accordance with Advanced Powder Metallurgy (APM), such material containing about 73% Fe, 22% Cr and 5% Al. The powder is extruded in a tubular form.
However, NOx concentrations increase significantly in waste gases at such high temperatures.
It is known to purify the gases originating from gas burners catalytically in a laboratory environment. The gases are caused to pass through a three-way catalyst that includes ceramic monolith. The NOx concentrations are herewith reduced by bringing the lambda value (L) to beneath L=1. However, this results in a significant increase in the CO-content of the gas instead.
One problem encountered at high temperatures is that the catalyst is subjected to high thermal stresses, which are liable to destroy a conventional catalyst that includes a ceramic monolith unless the catalyst is cooled.
The present invention solves these problems and enables much lower concentrations of CO, NOx and HC to be achieved than those achieved with conventional catalytic purification.
This enables a gas burner to operate at higher temperatures without increasing the load on the environment. Contrary to increasing the load on the environment, the concentrations of said substances will be significantly lower at high temperatures than the concentrations obtained with a conventional burner at typically lower temperatures.
A significant advantage afforded by the present invention is that it can be applied in a corresponding manner to lower the concentrations of HC, CO and NOx in the exhaust gases of internal combustion engines, such as automobile engines.
The present invention thus relates to a method of cleansing from waste gases, or from exhaust gases that originate from burners and internal combustion engines, their nitrogen oxide (NOx), hydrocarbon (HC) and carbon monoxide (CO) contents. The waste gases, or exhaust gases, are led through a catalyst for catalytic cleansing of the gases. The method includes lowering the lambda value (L) to a level beneath L=1. The gases are led through a first catalyst and then through a second catalyst. The CO-content of the gas in the first catalyst is brought to a sufficiently high level to reduce NOx to N2 to an extent such as to bring said NOx content to a predetermined NOx level. Sufficient oxygen (O2) is delivered to the gas between the first and the second catalyst to oxidize both CO and HC to CO2 and H2O to an extent such as to reduce the CO-content of the gas to a predetermined HC level.
The invention also relates to a gas burner that includes a catalytic gas purification or cleansing facility of the aforesaid kind.